Appendix A contains the following files in one CD-ROM (of which two identical copies are attached hereto), and is a part of the present disclosure and is incorporated by reference herein in its entirety:
Volume in drive D is 020129xe2x80x941155
Volume Serial Number is E6C3-5E3C
Directory of d: 
1/29/02 11:55a  less than DIR greater than 
1/29/02 11:55a  less than DIR greater than 
1/29/02 11:47a 65,024 $0PN01!.DOC
3 File(s) 65,024 bytes
Total Files Listed:
3 File(s) 65,024 bytes
0 bytes
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.
1. Field of the Invention
This invention relates to communications systems such as modems and to methods for detecting or characterizing digital impairment that occurs when communicating via a telephone network.
2. Description of Related Art
ITU standard V.90 defines 56K modems use of digital and PCM (pulse code modulated) signals for downstream communication from a service provider to a home user. For such communications, the telephone system includes a digital network that carries a digital signal, a PCM codec that converts the digital signal to a PCM signal, and telephone wires that bring the PCM signal to the downstream modem. In interpreting a received signal, 56K modems must have a precise model of the PCM codec at the digital/analog network interface and a precise model of the digital impairment over the digital telephone network. Generally, modems communicating via a telephone network can easily identify or distinguish the codec type, e.g., A-law or xcexc-law, that the telephone network uses, but identifying or distinguishing among every kind of digital impairment of connections can be challenging. 56K modem that fails to identify or learn the digital impairment, can suffer a 30% performance penalty because for accuracy, data transmission must be limited to the worst case scenario. Accordingly, a modem must correctly learn and adjust to the possible digital impairments to provide optimal performance and be commercially successful.
The common digital impairments introduced in telephone networks are well known. For example, for robbed-bit signaling (RBS), networks periodically use the least significant bit (LSB) of a PCM codeword for network control purposes. After using the robbed bits from PCM codewords, the network sets robbed bits to all zeros (even RBS), all ones (odd RBS), or alternates between zero and one (even-odd RBS). Even, odd, and even-odd RBS use the least significant bit of every sixth PCM codeword, but even-odd RBS has a period of twelve codewords because of the alternating replacement of the robbed bit with 1 and 0. RBS may also occur at more than one RBS phase due to multiple signaling in the digital network. Another kind of RBS, called middle RBS, uses a codec that maps each robbed PCM symbol to a level between the level for the PCM symbol with the LSB set to zero and the level for the PCM symbol with the LSB set to one. This document refers to even-odd RBS and middle RBS as strange RBS. Another type of digital impairment known as a digital pad, uses a look-up table that converts each PCM codeword to another PCM codeword to attenuate a signal on a channel.
Typically, the digital impairment can be modeled as a combination of RBS before a digital pad, then the digital pad and then RBS after the digital pad. A middle RBS never happens before a digital pad because middle RBS is implemented at the codec. Even-odd RBS is usually after digital pad. A general model of digital impairment includes six (or twelve) look-up tables, one for each RBS phase. Each look-up table represents the mapping of input PCM codewords to output values during the RBS phase associated with the look-up table.
A modem needs to learn or identify the above-described the digital channel impairments during handshaking to optimally selected a highest possible bit rate that can be accurately transmitted over a channel. However, analog channel impairment makes learning or identifying the digital impairment more difficult. Dealing with the analog impairments is the kernel problem of digital impairment learning.
In accordance with an embodiment of the invention, short pseudo-random (PR) probing sequences that comply with ITU V.90 digital impairment learning (DIL) descriptors form DIL probing sequences. The short PR probing sequences include subsequences associated with specific codes. Each subsequence contains products of the associated code and a pseudo-random series of values +1 and xe2x88x921. The pseudo-random nature of the short PR sequences cancels inter-symbol interference (ISI) so that the probing sequences do not require the insertion of extra zero symbols to remove ISI. Additionally, the DIL probing sequences yield high performance in severe inter-symbol interference (ISI) channels.
In accordance with a further aspect of the invention, a novel receiving structure corrects for equalizers that propagate of digital impairment among symbols. The receiving structure can achieve an ISI free receipt of the designed probing sequence within the strict time constraints of the ITU V.90 modem standard. The correction process solves a system of equations based on the wrapped channel response. The wrapped channel response can be determined from information obtained during training of an equalizer for the channel. General digital impairment mapping tables, digital pads, regular and strange RBS patterns, and different types of PCM codecs (A-law/xcexc-law) can be identified from signals reliably received through the receiving structure.
In accordance with one embodiment of the invention, a DIL process includes receiving a series of samples of a probing signal transmitted through a channel and identifying a set of the samples that corresponds to a selected code. The set corresponds to repeated transmission of the code over the channel, wherein each repetition of the first code has a sign from a pseudo-random series. The DIL process then determines a plurality of averages of the samples from the set. Each average corresponds to an associated phase of robbed bit signaling that occurs in the channel. From the averages, the DIL process can identify the digital impairment in the channel. One method of identifying the digital impairment determines from the averages, specific codes output from a digital network in the channel when the selected code is input to the channel. Determined output codes for the selected code and other input codes provide measured points in a set of mappers that maps input codes to output codes. To find a set of complete mappers for the channel, the measured points can be matched with corresponding points in predetermined maps from a library stored in a memory.
To determine the output codes from the averages, the process includes: determining a scaling factor for the channel; identifying the pulse code modulation (PCM) decoding employed in the channel; scaling the averages by the scaling factor to generate scaled averages; and encoding the scaled averages using an encoding method that corresponds to the identified PCM decoding. The encoded and scaled averages indicate the measured points for the channel""s mappers.
Determining the averages may include: determining a plurality of initial averages and then correcting the initial averages. Each initial average is an average of samples corresponding to an associated phase of the robbed bit signaling, and correcting the initial averages corrects for propagation of the digital impairment by an equalizer. To correct for the equalizer, the process determines a wrapped channel response for the equalizer and solves system of equations for the corrected averages. The system of equations equates a vector containing the initial averages with a product of a matrix derived from the wrapped channel response and a vector containing the averages.
In accordance with another embodiment of the invention, a process sends a novel probing signal for detection of digital impairment in a channel. The probing signal corresponds to a probing sequence that includes one or more subsequences. Each subsequence includes repetitions of an associated code, and each repetition is multiplied by an associated value from a pseudo-random series of +1 and xe2x88x921. The pseudo-random sign for the subsequences cancels ISI. In accordance with another aspect of the invention, an equalization process for digital impairment learning corrects for feedback terms of an equalizer that propagate digital impairment among received values.
Another embodiment of the invention is a communication system that implements any of the processes described herein. Once such system includes a receiver with a mean value estimator, a coding identifier, a scaling estimator, a memory, and a map set identifier. The mean value estimator receives a series of samples representing a probing signal transmitted over a channel and determines a plurality of averages. Each average corresponds to a phase of robbed bit signaling on the channel. The coding identifier receives and processes the averages to identify a coding process (e.g., xcexc-law or A-law) used in the channel. The scaling estimator receives and processes the averages to identify a scaling factor for the channel. The memory stores entries corresponding to a library of mapper sets with each mapper set defining a correspondence between input codes and output codes from a digital network. The map identifier uses information from the mean value estimator, the scaling estimator, the coding identifier, and the memory to identify an entry in the memory that matches the averages as adjusted for the scaling factor and the coding process. The entry identified indicates the digital impairment in the channel.